Here, we suggest that two-to-two dark matter fusion may be the relaxation process that resolves the small-scale structure problems of the cold collisionless dark matter paradigm. In order for the fusion cross section to scale correctly across many decades of astrophysical masses from dwarf galaxies to galaxy clusters, we require the fractional binding energy released to be greater than v n~(10 –(2–3)) n, where n=1, 2 depends on local dark sector chemistry. The size of the dark-sector interaction cross sections must be σ~0.1–1 barn, moderately larger than for standard model deuteron fusion, indicating a dark nuclear scale Λ~O(100 MeV). Darkmore » fusion firmly predicts constant σv below the characteristic velocities of galaxy clusters. Observations of the inner structure of galaxy groups with velocity dispersion of several hundred kilometers per second, of which a handful have been identified, could differentiate dark fusion from a dark photon model.« less

A new understanding of the stability of self-interacting dark matter is pointed out, based on the simplest spontaneously broken Abelian gauge model with one complex scalar and one Dirac fermion. The key is the imposition of dark charge conjugation symmetry. It allows the possible existence of two stable particles: the Dirac fermion and the vector gauge boson which acts as a light mediator for the former's self-interaction. Since this light mediator does not decay, it avoids the strong cosmological constraints recently obtained for all such models where the light mediator decays into standard-model particles.